1. Field of the Invention
The current invention relates to devices made from narrow-gap (and more conventional) semiconductors for operation at infrared wavelengths.
2. Discussion of Prior Art
Many prior art devices include a diode junction, hence metal contacts need to be formed to a region of n-type and a region of p-type material. Ideally the metal contact should be ohmic and have a low resistance, i.e. very much less than that of the junction. The surfaces of the semiconductors tend to comprise a native oxide, which is normally insulating to some extent and must be removed or disrupted to permit contact to the semiconductor underneath. The nature of the band-bending which often occurs at the metal-semiconductor interface means that it is relatively easy to obtain the desired contact to n-type material, but it can be difficult to achieve an ohmic, low resistance contact to the p-type semiconductor.
Alloying using a metal with a small proportion of a like-type dopant can be used with some semiconductors. When annealed, this gives a heavily doped region to which the metal is more likely to form an ohmic contact, for example Au/Zn for a p.sup.+ p contact to GaAs. This does not work well, however, with the narrow-gap semiconductors, such as indium antimonide (InSb) and cadmium mercury telluride (CMT) because of their low melting temperatures and the high diffusion rates of some of the dopants. Ion milling is often used for the narrow-gap materials to remove the surface oxide. This tends to result in near surface (typically tens of nanometers) damage which is heavily n-type in nature plus additional damage which can propagate significantly further into the material and degrade the junction properties. As a consequence the thickness of the contact regions in diodes made from the narrow-gap semiconductors needs to be greater than, or of the order of, 10.sup.-6 m.
In devices such as light emitting diodes and diode lasers where optical radiation passes through one or both contact regions, this thickness of material can lead to significant optical losses which will degrade the device performance, in some cases catastrophically.
Degenerate doping of the material, i.e. doping to a sufficiently high level that the Fermi energy lies within the appropriate band, leads to a Moss-Burstein shift whereby the absorption coefficient for radiation of energy just greater than the band-gap decreases so that it becomes transparent. Owing to the band structure of the narrow-gap semiconductors, however, it is extremely difficult to induce degenerate doping levels in p-type material, so that a sufficient Moss-Burstein shift is not observable. In contrast, it is comparatively easy to produce degenerate doping in n-type material and large Moss-Burstein shifts can be achieved. Hence the n-type contact regions of diodes can be made transparent to radiation generated or absorbed in the active region but the p-type contact region can not.
Also relevant to the present invention is U.S. Pat. No. 5,338,944. This document describes a light emitting diode for emitting visible radiation in the blue region of the visible spectrum, comprising an n-type silicon carbide substrate, an n-type silicon carbide top layer and a light emitting p-n junction structure between the n-type substrate and the n-type top layer. The device also includes, between the n-type top layer and the n-type substrate, means for coupling the n-type top layer to the light emitting p-n junction structure while preventing n-p-n behaviour between the n-type top layer, the p-type layer in the junction structure and the n-type substrate. This is achieved by means of a degenerate junction structure comprising a p-type portion and an n-type portion of silicone carbide. The p-type portion and the n-type portion are very thin, of the order of 250-1000 angstroms, with a very high doping concentration of at least 1.times.10.sup.-19 cm.sup.-3. This invention relates to light emitting diodes operating in the blue region of the visible wavelength spectrum. It therefore relates to devices formed from materials having a bandgap of at least 2.6 eV, such as silicon carbide.
Applied Physics Letts, 62(1993) 17 May, No. 20, New York, USA (A. R. Sugg et al.) relates to n-p-(p.sup.+ -n.sup.+)-n Al.sub.y Ga.sub.1-y As-GaAs-In.sub.x Ga.sub.1-x As quantum-well laser with p.sup.+ -n.sup.+ GaAs-InGaAs tunnel contact on n-GaAs. Hence, this document relates to laser devices formed on GaAs substrates.